1. Technical Field
The present disclosure relates to a control device for a resonant converter.
2. Description of the Related Art
Switching converters with devices used for the control thereof are known in the state of the art. Resonant converters represent a wide range of switching converters and include a resonant circuit playing an active role in determining the input-output power flow. In these converters, a bridge (half-bridge) consisting of four (or two) power switches (typically power MOSFETs) supplied by a DC voltage generates a square wave voltage that is applied to a resonant circuit tuned to a frequency close to the fundamental frequency of said square wave. Thereby, because of the selective features thereof, the resonant circuit mainly responds to the fundamental component and negligibly to the higher order harmonics of the square wave. As a result, the circulating power may be modulated by varying the frequency of the square wave, holding the duty cycle constant at 50%. Moreover, depending on the resonant circuit configuration, the currents and/or voltages associated with the power flow have sinusoidal or piecewise sinusoidal shape.
These voltages are rectified and filtered so as to provide DC power to the load. In offline applications, to comply with safety regulations, the rectification and filtering system supplying the load is coupled with the resonant circuit by means of a transformer providing the isolation between source and load to satisfy the above-mentioned regulations. As in all isolated network converters, also in this case a distinction is made between a primary side (as related to the primary winding of the transformer) connected to the input source and a secondary side (as related to the secondary winding(s) of the transformer) providing power to the load through the rectification and filtering system.
Among the many types of resonant converters, the so-called LLC resonant converter is widely used, especially in the half-bridge version thereof. The designation LLC comes from the resonant circuit employing two inductors (L) and a capacitor (C); a principle schematic of an LLC resonant converter is shown in FIG. 1. The resonant converter 1 comprises a half-bridge of transistors Q1 and Q2 comprised between the input voltage Vin and ground GND and controlled by a control circuit 3. The common terminal HB between the transistors Q1 and Q2 is connected to a circuit block 2 comprising a series of a capacitor Cr, an inductance Ls and another inductance Lp connected in parallel to a transformer 10 with a center-tap secondary. The two windings of the center-tap secondary of the transformer 10 are connected to the anodes of two diodes D1 and D2 the cathodes of which are both connected to the parallel of a capacitor Cout and a resistance Rout; the output voltage Vout of the resonant converter is across said parallel while the DC output current Iout flows through Rout.
Resonant converters offer considerable advantages as compared to traditional switching converters (not resonant, typically PWM—Pulse Width Modulation—controlled): waveforms without steep edges, low switching losses in the power switches due to the “soft” switchings thereof, high conversion efficiency (>95% is easily reachable), ability to operate at high frequencies, low EMI generation (Electro-Magnetic Interference) and, ultimately, high power density (that is, enabling to build conversion systems capable of handling considerable power level in relatively small space).
As in most DC-DC converters, a closed-loop negative feedback control system keeps the output voltage of the converter constant upon changing the operating conditions, that is the input voltage Vin and/or the output current Iout thereof. This is achieved by comparing a portion of the output voltage with a reference voltage; the difference or error signal between the value provided by the output voltage sensing system (usually, a resistor divider) and the reference value is amplified by an error amplifier the output of which modifies a quantity x within the converter and which the energy carried by the converter during each switching cycle substantially depends on. In resonant converters, such a significant quantity is the switching frequency of the square wave stimulating the resonant circuit.
A desire common to many applications of the switching converters and, therefore, also to those in which resonant converters are used, is of optimizing the conversion efficiency (that is the ratio between the output power and input power) also at low loads and/or minimizing the power drawn from the source when the load is null, to comply with the regulations on energy saving (e.g., EnergyStar, CEC, Eu CoC, etc.).
A technique widely implemented in all switching converters (resonant and not) for optimizing the efficiency at low load and minimizing the power absorbed with zero load is to operate the switching converters in the so-called “burst-mode”. In this operation mode, the converter operates intermittently with series of switching cycles (bursts) separated by time intervals in which the converter does not switch. When the load is so low that the switching converter operates in burst mode, the intervals in which the converter does not switch are initially quite short; as the load decreases these intervals get longer: the duration of the bursts decreases and their distance in time increases. Thereby, the average switching frequency is considerably decreased and so are the losses associated with the switching of the parasitic elements of the converter and with the flow of the magnetizing current of the transformer which represent the majority of the losses of power under low or very low load conditions.
In all known embodiments, the entry into such burst mode occurs when the transiting power decreases below a pre-established level. The same feedback control loop controls the succession of the bursts so that the output voltage of the converter always remains under control.
In Pulse-Width-Modulation-controlled (PWM) converters, by virtue of the control methods used, there is a direct relationship between the power level which transits in the converter and the control quantity, hence the burst-mode operation is simply provided by the use of a comparator with hysteresis, in the manner described hereinafter.
When the control variable is less than a pre-established threshold the converter is turned off. Due to the stopping of the energy flow, the output voltage starts to decrease slowly, because the load is low. The feedback loop reacts to this lowering of the output voltage by increasing the control voltage and, when this exceeds the aforesaid threshold by a quantity equal to the hysteresis, the converter turns on again. Due to this, the output voltage increases and, consequently and again due to the feedback loop, the control voltage decreases once more. As soon as said voltage returns below the pre-established threshold the converter is turned off again, and so on.
In all control integrated circuits for resonant DC-DC converters on the market, the control directly operates on the oscillation frequency of the half-bridge (Direct Frequency Control, DFC).
In the control systems for resonant converters the burst-mode operation is implemented in the same mode as in the PWM controllers, that is by comparing the control voltage with a reference in a comparator with hysteresis. FIG. 3 shows a circuit which implements the burst-mode operation of the integrated control circuit L6599 by STMicroelectronics. In this device the switching frequency is determined by a current-controlled oscillator (CCO) adapted to drive the transistors Q1 and Q2 by the signal Drive, which is programmed by means of the resistors R1 (which sets the minimum operating frequency when the current flowing in the phototransistor of the optical coupler is null) and R2 (which determines the frequency at which the device enters in burst mode) which, together with the reference voltage Vr available at the pin, define the charge/discharge current of the timing capacitor C1 connected to the pin CF. When the current derived from the phototransistor TC is such that the voltage on the collector terminal thereof, brought to pin B, is less than the threshold voltage Vh, the output of the comparator CO9 goes high thus inhibiting the oscillator CCO and turning off both the switches Q1 and Q2, thus turning off the half-bridge. When, due to the reaction of the control loop, such a current is decreased so that the voltage at the pin B exceeds the threshold Vh by a quantity equal to the hysteresis of the comparator CO9, the output thereof goes low thus retriggering the oscillator CCO and determining the restart of the transistors Q1 and Q2 and, therefore, of the half-bridge.
In other controllers available on the market such a function is provided in similar way.
Since the controlled quantity is the frequency and in a resonant converter the frequency increases as the load decreases, a converter employing said integrated controller will enter into burst-mode operation when the operating frequency thereof exceeds a prefixed value (programmed by the resistance R2 in FIG. 3).
Unfortunately, the frequency in resonant converters does not only depend on the load but also, and especially, on the input voltage. On the contrary, in a feedback-controlled resonant converter, the switching frequency changes more due to variations of the input voltage than due to load variations. Another problem is that the operating frequency for assigned conditions of input and load voltage may have considerable variations, due to the statistical dispersion of the characteristic parameters of the resonant circuit (Cr, Ls and Lp in FIG. 1) due to their tolerances. The result is that the power level at which the converter operates at the frequency of the burst-mode operation may have considerable variations and a low production repeatability depending on the input voltage variations and the aforementioned parametric dispersion effect. All this is clearly visible in the diagrams in FIGS. 4A and 4B, in which there is shown the dependency of the frequency f (in kHz) on the input voltage Vin for fixed values of the output power Pout, i.e., when the load varies by a percentage from 1 to 10%, in a reference LLC resonant converter (FIG. 4A) and the dependency of the frequency f on the output power Pout i.e., when the load varies by a percentage from 1 to 12%, for fixed values of the input voltage Vin (FIG. 4B), comprised between 300V and 440V, again in a reference LLC resonant converter.